Folds & Faults
Topics covered in this section:
Structural Geology - Folding of Rocks
Folds are the second important component of fold-thrust belts and rock deformation in general. Whereas faults were treated as a general manifestation of brittle deformation within the upper 10 km of the crust, folds are generally regarded as a manifestation of ductile deformation. In fold-thrust belts folds are necessary to accommodate faulting. Folds are most commonly marked by bedding, a surface referred to as S0 in structural geology. Folds are characterized by axial surfaces, S1, and a fold axis (Fig. 1). The plane perpendicular to the fold axis is that surface on which the profile of a fold is projected. A fold that has a planar axial surface is said to be a plane fold, otherwise it is a nonplanar fold.
One ideal fold is cylindrical with any profile the same as any other profile (Fig. 2). Real folds are rarely cylindrical but rather conical in shape. The ideal fold can be divided into the hinge area where the radius of curvature of the fold is smallest and the limb area where the radius of curvature of the fold is largest. Figure 2 shows a profile or cross section through a set of cylindrical folds constituting a wave train. The Amplitude and wave length of the wave train are shown.
Anticlinal structure with multipy plunging hinge lines, obliquely cross-cut by a thrust fault
Folds in GeoModeller
Surfaces of geology units can be impacted by fold structures. Folds can be defined fundamentally in a 2D section by the trace of contact data around a hinge area, together with orientation data for the folded limbs.
First create an axial series
Next, create an axial surface ‘object’ (linked to an axial series)
Define anticline or syncline
What is now defined?
What’s a fold axis / hinge line?
What’s a fold axial plane?
Next, create axial surface data, on a 2D section (& assign it). By digitising, use the short-cut, or menu
Next, create axial surface orientation data, on a section by digitising, use the short-cut, or menu
After this set-up, “Compute” will now succeed (because of ordinary contact and orientation data – for our 2 layers), but no account will yet be made of our fold axial data, because no fold shape parameters are yet defined
Next: define an axial surface section
Note: these axial Surface sections can be non-planar (eg., like a topography section)
Create Fold Shape Parameters
Create hinge line data on the fold axial surface section (defined by geology contact points associated with a formation). Define a hinge line with at least two contact points.
Constant Fold Shapes
The shape of the fold at the hinge line is defined by a set of points that are described by their position along the hinge line (digitised), and by two additional points contained in a plane perpendicular to the axial plane (in the profile plane)
These points are symmetrical about the fold axial plane, unless additional (standard contact data) control the fold shape
The positions of the additional points are determined by a distance away from the fold axis, and a vector. Different constant-fold shapes, and the hinge line data that define them are shown:
Non-Constant Fold Shapes
Can also be defined in GeoModeller:
Faults in GeoModeller
Both infinite and finite faults can be controlled:
First create the fault ‘object’:
Until ‘Attributes’ are defined, the faults will be infinite
Allow the “link faults with series editor’ to start automatically
To define a fault surface, Add:
LETS see the geology data already in this project. Reference: Tops. Now Compute
Next: Fault data were added for F2. And more red_series contact points added in the new fault compartment area, then re-computed
Re-cap: Overview in 3D
Now lets say: F1 is older, and is cross-cut by F2 (high-angle reverse fault)
MENU: Geology > Link faults with faults. Now re-compute
F1 stops on F2
F1 can’t be shown displaced by F2 at present - (A displaced segment would need to be drawn as a separate fault)
Finite or Infinite Faults in the project cube?
Lets make F1 a finite fault in the project cube – with limited active displacement
Next, make sure the new limits of F1 will interact with data for the red_series (otherwise it won’t compute). F1 now defined on Section250. Now compute
F1 is now “limited”. F1 still stops on F2
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